Several dispensers of the kind specified are known, some of which function by injecting a galvanic element into an electrolyte (see for example GB-A-1102666). Others use electric cells to pass current through electrolytes to generate the dispensing gas (see for example GB-A-1496841 and WO 93/25841). In these known devices, the gas generated is contained in an expansion chamber and expands against a piston to expel the fluent lubricating material into an appropriate receptacle, e.g. a bearing.
Dispensers of the kind specified normally contain sufficient motive power-source materials to generate the appropriate quantity of gas to expel all the fluent lubricating material with only a small surplus of the power-source materials. This surplus is deliberately restricted to ensure that the dispenser does not become over-pressurised after it is spent and the piston has come to the end of its stroke. The normal operating gauge pressure within a dispenser of the kind specified is in the range from about 0.5 bar to about 3 bar depending on the resistance offered to the flow of material leaving the dispenser, e.g. the resistance caused by channels into which a lubricant is injected. Even when the piston reaches the end of its stroke, having expelled all the fluent material, the expanded gas chamber is sufficiently large to ensure that any surplus gas generated does not materially increase the pressure (and certainly does not rise to a dangerous level).
However, it can happen that a dispenser of the kind specified can be fitted unwittingly to, for example, a lubrication point in which the lubricant channels have become blocked by dirt or hardened grease. If the pressure generated by the dispenser is insufficient to force the lubricant past the obstruction, the piston will be unable to move as the lubricant in the dispenser will be substantially incompressible and the expansion chamber, which will normally have very little free gas space initially, will be unable to expand. As the galvanic reaction continues and gas continues to be generated, the gas pressure inside the dispenser can rise to an unsafe level, e.g. in excess of 10 bars gauge, giving rise to the possibility of a dangerous explosive burst.